Five women volunteers will measure their resting alveolar PCO2 levels before and during breathing 5% CO2. This value will be correlated with their venous progesterone levels at various stages of their menstrual cycle. One pregnant women will also measure her CO2 levels and progesterone levels over the course of the study. The hypothesis is that end tidal PCO2 levels and CO2 response curves are affected by circulating levels of progesterone.